Long Term Evolution (LTE) is an evolution of Universal Mobile Telecommunications System (UMTS), standardized by an international standardization body, 3rd Generation Partnership Project (3GPP). The configuration of an LTE system is illustrated in FIG. 1.
FIG. 1 is a view referred to for describing the configuration of an LTE system.
The LTE system may be divided largely into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN includes UEs and evolved Node Bs (eNBs). A UE is connected to an eNB via a Uu interface and one eNB is connected to another eNB via an X2 interface. The EPC includes a Mobility Management Entity (MME) responsible for control-plane (C-plane) functions and a Serving GateWay (S-GW) responsible for user-plane (U-plane) functions. An eNB is connected to the MME via an S1-MME interface and an eNB is connected to the S-GW via an S1-U interface. These two interfaces are collectively called an S1 interface.
For the Uu interface being an air interface, a radio interface protocol stack is defined. The radio interface protocol stack horizontally includes a PHYsical (PHY) layer, a data link layer, and a network layer and vertically includes a U-plane for user data transmission and a C-plane for control signaling. Based on the lowest three layers of the Open System Interconnection (OSI) reference model known in communication systems, this radio protocol stack can be divided into Layer 1 (L1) including a PHY layer, Layer 2 (L2) including a Medium Access Control/Radio Link Control/Packet Data Convergence Protocol (MAC/RLC/PDCP) layer, and Layer 3 (L3) including a Radio Resource Control (RRC) layer. These layers are defined in pairs between a UE and an Evolved UTRAN (E-UTRAN), for data transmission via the Uu interface.
Now a description will be given below of a Long Term Evolution Advanced (LTE-A) system.
LTE-A is a system developed from LTE to meet the 4th Generation (4G) mobile communication requirements, that is, IMT-Advanced requirements recommended by the International Telecommunication Union-Radio communication sector (ITU-R). The 3GPP which developed the LTE system standard is now actively working on standardization of the LTE-A system.
Major technologies added to the LTE-A system are carrier aggregation for extending a used bandwidth and flexibly using the bandwidth and use of relays for supporting group mobility and enabling user-centered network deployment.
FIGS. 2 and 3 are views referred to for describing radio protocol layers.
At L1, the PHY layer provides information transfer service to its higher layer on physical channels. The PHY layer is connected to the MAC layer through transport channels and data is transferred between the MAC layer and the PHY layer on the transport channels. The transport channels are largely divided into dedicated transport channels and common transport channels depending on whether the transport channels are shared or not. Data is transmitted on physical channels using radio resources between different PHY layers, that is, the PHY layers of a transmitter and a receiver.
There are a plurality of layers at L2. The MAC layer maps logical channels to transport channels and performs logical channel multiplexing by mapping a plurality of logical channels to one transport channel. The MAC layer is connected to its higher layer, the RLC layer through logical channels. Depending on the types of information carried on the logical channels, the logical channels are classified into control channels that deliver C-plane information and traffic channels that deliver U-plane information.
The RLC layer at L2 adjusts a data size to be suitable for data transmission in the air interface from a lower layer by segmenting and concatenating data received from a higher layer. In order to guarantee various Quality of Service (QoS) requirements of each Radio Bearer (RB), the RLC layer provides three operation modes, Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). Especially, an AM RLC performs a retransmission function through ARQ, for reliable data transmission.
The PDCP layer at L2 compresses a header to reduce the size of an Internet Protocol (IP) packet header of a relatively large size containing unnecessary control information to efficiently transmit an IP packet such as an IPv4 or IPv6 packet via a radio link having a narrow bandwidth. The header compression function enables transmission of necessary information in a header only, thereby increasing the transmission efficiency of a radio link. In addition, the PDCH layer performs a security function in the LTE system. This security function involves ciphering for preventing a third part from eavesdropping and integrity protection for preventing a third party from maliciously modifying data.
The RRC layer at the highest of L3 is defined only on the C-plane. The RRC layer takes charge of controlling logical channels, transport channels, and physical channels in relation to configuration, reconfiguration, and release of RBs. An RB is a logical path provided by L1 and L2 in the radio protocol architecture, for data transmission between a UE and a UTRAN. In general, configuring an RB means defining the features of a radio protocol layer and channels needed to provide a specific service and setting specific parameters and an operation scheme. RBs are classified into Signaling RB (SRB) and Data RB (DRB). The SRB is used as a path in which an RRC message is transmitted on the C-plane and the DRB is used as a path in which user data is transmitted on the U-plane.
Now, a description will be given of RRC_IDLE state of a UE. In the RRC_IDLE state, the UE should always select a cell having a suitable quality and prepare for receiving a service from the selected sell. For example, upon power-on, the UE should select a cell having a suitable quality to register to a network. If the UE transitions from RRC_CONNECTED to the RRC_IDLE state, it should select a cell to camp on. The process of selecting a cell satisfying a specific condition to stay in a service idle state such as the RRC_IDLE state is called cell selection.
How a UE selects a cell will be described in detail. Upon initial power-on, the UE searches available Public Land Mobile Networks (PLMNs) and selects a suitable PLMN from which to receive a service. Subsequently, the UE selects a cell having a signal quality and characteristics good enough to provide a service to the UE. Cell selection is largely divided into two processes. One is initial cell selection. During initial cell selection, the UE has no prior knowledge of radio channels. Therefore, the UE scans all radio channels to search for a suitable cell. Once the UE detects a suitable cell satisfying a cell selection criterion, it selects the detected cell. The other cell selection process is cell selection. During cell selection, the UE selects a cell based on stored information about radio channels or information broadcast from cells. Accordingly, cell selection can be faster than during initial cell selection. Once the UE detects a cell satisfying the cell selection criterion, it selects the detected cell. If the UE fails to detect a suitable cell satisfying the cell selection criterion, it performs initial cell selection.
How a UE reselects a cell will be described below. After the UE selects a cell through cell selection, signal strength or quality between the UE and an eNB may change due to a change in the mobility of the UE or a change in radio environment. If the quality of the selected cell is degraded, the UE may select another cell offering a better quality. When reselecting a cell in this manner, the UE generally selects a cell having a better signal quality than the current cell. This process is called cell reselection. The basic purpose of cell reselection lies in selection of a cell having the best quality. Aside from the aspect of radio signal quality, the network may prioritize frequencies and notify the UE of the priority levels of the frequencies. Then, the UE puts the priority levels of frequencies before the qualities of radio signals during cell reselection.
Now a description will be given of CA in the LTE-A system.
FIG. 4 is a view referred to for describing CA.
As described above, the LTE-A standard is designed as an IMT-Advanced candidate technology of the ITU to satisfy IMT-Advanced technical requirements. Accordingly, extension of a bandwidth from the legacy LTE system is under discussion to satisfy IMT-Advanced technical requirements. For bandwidth extension, carriers available to the legacy LTE system are defined as Component Carriers (CCs) in the LTE-A system. Aggregation of up to 5 CCs is under discussion, as illustrated in FIG. 4. Because a CC may occupy up to 20 MHz as in the LTE system, the CA technology of the LTE-A standard is a concept of extending a bandwidth to up to 100 MHz. The technology of aggregating a plurality of CCs is called CA.